The need to protect metal surfaces, particularly the surfaces of ferrous metals, during storage between operations is recognized throughout the industry. It is not uncommon for metal coils, for example, to be stored for prolonged periods prior to the next working operation, which can be stamping or another rolling operation to further reduce the thickness of the sheet and/or impart the desired metallurgical properties to the metal. Very often the metal coils are stored in areas adjacent to pickling lines and as a result the metal is exposed to the corrosive action of acid vapors. To prevent, or at least minimize, deterioration of the metal surfaces protective coatings are commonly applied.
Numerous types of protective coatings are known and have been reported in the literature to protect against rusting. Typically these products form a continuous hydrophobic barrier on the metal surface, however, while they may be impervious to moisture they are not totally effective in the presence of corrosive vapors. Accordingly, it is necessary to formulate products which are specifically designed to withstand the rigorous conditions encountered when substantial concentrations of acidic vapors are present.
U.S. Pat. No. 4,166,151, for example, discloses "waxy" ester compositions useful for protecting metal surfaces from acidic vapor which are derived from C.sub.10-25 aliphatic carboxylic acids and C.sub.15-40 aliphatic alcohols. These ester products are applied to the metal in an inert, volatile hydrocarbon, however, many of the waxy esters have limited solubility in the commonly used hydrocarbon carriers and must be applied as dispersions, which can present problems. For example, if the dispersions are allowed to stand, agitation is necessary prior to use and may even be required throughout the application period, depending on the type of equipment used. It is also difficult to obtain uniform deposition over the entire surface area of the metal so that after the hydrocarbon solvent has evaporated, discontinuities may be present in the waxy protective film. Areas which are deficient or totally devoid of the waxy protective coating will, of course, be susceptible to the corrosive action of the acid vapors.
The use of coating oil formulations, such as those of U.S. Pat. No. 3,600,310, which contain waxes or wax-like products can also pose problems in subsequent rolling operations if aqueous rolling oils are employed. These waxy coating materials are removed, at least partially, during rolling and become associated with the aqueous rolling oil. The wax, being incompatible with the aqueous system, results in emulsion instability, application problems and other associated difficulties, particularly as the amount of wax in the aqueous rolling oil increases in the system upon recirculation. The resulting wax buildup will coat equipment, clog nozzles and give heavy localized wax deposits on the metal which can result in undesirable carbonaceous deposits if the metal is subjected to heat treatment or annealing.
Slushing oil or rust inhibiting compositions are disclosed in U.S. Pat. No. 3,857,789 which are comprised of a Group IIA, IIIA or IVA metal salt of a mahogany or naphthalene sulfonic acid, an aliphatic carboxylic acid, a cosolvent selected from aromatic hydrocarbons, methyl pyrrolidone, tetrahydrofuran, and mono and dialkyl ethers of alkylene glycols and mixtures thereof, and a paraffinic or naphthenic lubricating oil. Whereas these compositions are effective for preventing corrosion of iron objects, the use of costly aromatic or heterocyclic cosolvents is necessary if homogeneous solutions, which do not separate upon standing, are to be obtained.